Conventional cardiopulmonary bypass uses an extracorporeal blood circuit that is to be coupled between arterial and venous cannulae and includes a venous drainage or return line, a venous blood reservoir, a blood pump, an oxygenator, an arterial filter, and blood transporting tubing or “lines”, ports, and valves interconnecting these components. Prior art, extracorporeal blood circuits as schematically depicted in FIGS. 1–3 and described in commonly assigned U.S. Pat. No. 6,302,860, draw venous blood of a patient 10 during cardiovascular surgery through the venous cannula (not shown) coupled to venous return line 12, oxygenates the blood, and returns the oxygenated blood to the patient 10 through an arterial line 14 coupled to an arterial cannula (not shown). Cardiotomy blood and surgical field debris that is aspirated by a suction device 16 is pumped by cardiotomy pump 18 into a cardiotomy reservoir 20.
Air can enter the extracorporeal blood circuit from a number of sources, including around the venous cannula, through loose fittings of the lines or ports in the lines, and as a result of various unanticipated intra-operative events. It is necessary to minimize the introduction of air in the blood in the extracorporeal blood circuit and to remove any air that does accumulate in the extracorporeal blood circuit before the filtered and oxygenated blood is returned to the patient through the arterial cannula to prevent injury to the patient. Moreover, if a centrifugal blood pump is used, a large volume of air accumulating in the venous line of the extracorporeal blood circuit can accumulate in the blood pump and either de-prime the blood pump and deprive it of its pumping capability or be pumped into the oxygenator and de-prime the oxygenator, inhibiting oxygenation of the blood.
In practice, it is necessary to initially fill the cannulae with the patient's blood and to prime (i.e., completely fill) the extracorporeal blood circuit with a bio-compatible prime solution before the arterial line and the venous return lines are coupled to the blood filled cannulae inserted into the patient's arterial and venous systems, respectively. The volume of blood and/or prime solution liquid that is pumped into the extracorporeal blood circuit to “prime” it is referred to as the “prime volume”. Typically, the extracorporeal blood circuit is first flushed with CO2 prior to priming. The priming flushes out any extraneous CO2 gas from the extracorporeal blood circuit prior to the introduction of the blood. The larger the prime volume, the greater the amount of prime solution present in the extracorporeal blood circuit that mixes with the patient's blood. The mixing of the blood and prime solution causes hemodilution that is disadvantageous and undesirable because the relative concentration of red blood cells must be maintained during the operation in order to minimize adverse effects to the patient. It is therefore desirable to minimize the volume of prime solution that is required.
In one conventional extracorporeal blood circuit of the type depicted in FIG. 1, venous blood from venous return line 12, as well as de-foamed and filtered cardiotomy blood from cardiotomy reservoir 20, are discharged into a venous blood reservoir 22. Air entrapped in the venous blood rises to the surface of the blood in venous blood reservoir 22 and is vented to atmosphere through a purge line 24. The purge line 24 is typically about a 6 mm ID flexible tubing, and the air space above the blood in venous blood reservoir 22 is substantial. A venous blood pump 26 draws blood from the venous blood reservoir 22 and pumps it through an oxygenator 28, an arterial blood filter 30, and the arterial line 14 to return the oxygenated and filtered blood back to the patient's arterial system via the arterial cannula coupled to the arterial line 14.
A negative pressure with respect to atmosphere is imposed upon the mixed venous and cardiotomy blood in the venous blood reservoir 22 as it is drawn by the venous blood pump 26 from the venous blood reservoir 22. The negative pressure causes the blood to be prone to entrain air bubbles. Although arterial blood filters, e.g., arterial blood filter 30, are designed to capture and remove air bubbles, they are not designed to handle larger volumes of air that may accumulate in the extracorporeal blood circuit. The arterial blood filter 30 is basically a bubble trap that traps any air bubbles larger than about 20–40 microns and discharges the air to atmosphere through a typically about 1.5 mm ID purge line 32. The arterial filter 30 is designed to operate at positive blood pressure provided by the venous blood pump 26. The arterial blood filter 30 cannot prevent accumulation of air in the venous blood pump 26 and the oxygenator 28 because it is located in the extracorporeal blood circuit downstream from them.
As shown in FIG. 2 from the above-referenced '860 patent, it has been proposed to substitute an assisted venous return (AVR) extracorporeal blood circuit for the conventional extracorporeal blood circuit of the type depicted in FIG. 1, whereby venous blood is drawn under negative pressure from the patient's body. The arterial blood filter 30 is moved into the venous return line 12 upstream of the venous blood pump 26 to function as a venous blood filter 30′. The venous blood reservoir 22, which accounts for a major portion of the prime volume of the extracorporeal blood circuit, is thereby eliminated. De-foamed and filtered cardiotomy blood from cardiotomy reservoir 20 is drained into the venous blood filter 30, and venous blood in venous return line 12 and the venous cannula coupled to it is pumped through the venous blood filter 30. Exposure of the venous blood to air is reduced because the venous blood filter 30′ does not have an air space between its inlet and outlet (except to the extent that air accumulates above the venous blood inlet), as the venous blood reservoir 22 does. Suction is provided in the venous return line 12 through the negative pressure applied at the outlet of venous blood filter 30′ by the venous blood pump 26 to pump the filtered venous blood through the oxygenator 28 and into the arterial blood line 14 to deliver it back to patient 10. Again, the venous blood filter 30′ is basically a bubble trap that traps any air bubbles larger than about 20–40 microns and discharges the air through a typically about 1.5 mm ID purge line 32.
The arterial blood filter 30 is also relocated with respect to the cardiotomy reservoir 20 and modified to function as a venous blood filter 30′ in the extracorporeal blood circuit shown in FIG. 3. Evacuation of air from venous blood received through venous return line 12 is facilitated by increasing the size of the purge port 34 of the venous blood filter 30′ to accept a larger diameter purge line 42, e.g. a 6 mm ID line, rather than the 1.5 mm ID line. A vacuum greater than that normally used for venous drainage is applied through purge line 42 to the purge port 34 to actively purge air from venous blood filter 30. The cardiotomy reservoir 20 is at ambient pressure but is conveniently purged by the same vacuum that purges air from venous blood filter 30. A valve 36, e.g., a one-way check valve, is incorporated into the purge port 34 or purge line 42 to prevent air or blood purged from the cardiotomy reservoir 20 from being drawn into venous blood filter 30′ by the negative pressure in venous blood filter 30′ when the purging vacuum is not active.
As shown in FIG. 4 from the above-referenced '860 patent, venous blood is drawn through the upper venous blood inlet 44 of venous blood filter 30′, down through the filter 46 and a screen or other conventional bubble trapping device (not shown), and out the venous blood outlet 48 by the venous blood pump 26. The purge port 34 is located above the venous blood inlet 44, and air that is separated out by the screen or other conventional bubble trapping device accumulates in the space 50 above the venous blood inlet 44. An air sensor 38 is disposed adjacent the purge port 34 that generates a sensor signal or modifies a signal parameter in the presence of air in the space 50. The sensor signal is processed by circuitry in a controller (not shown) that applies the vacuum to the purge line 42 to draw the accumulated air out of the space 50. The vacuum is discontinued when the sensor signal indicates that venous blood is in the space 50. Thus, an “Active Air Removal” (AAR) system is provided to draw the accumulated air out of space 50 when, and only when, air present in the space 50 is detected by air sensor 38 to purge the air and to prevent venous blood filling space 50 from being aspirated out the purge line 42 by the purging vacuum. The purging vacuum may be produced by a pump 40, or it may be produced by connecting the purge line 42 to the vacuum outlet conventionally provided in operating rooms.
Again, suction is provided in the venous return line 12 through the negative pressure applied at the outlet 48 of venous blood filter 30′ by the venous blood pump 26 to pump the filtered venous blood through the oxygenator 28 and into the arterial blood line 14 to deliver it back to patient 10. De-foamed and filtered cardiotomy blood is also pumped by venous blood pump 26 from cardiotomy reservoir 20 through the oxygenator 28 and into the arterial blood line 14 to deliver it back to patient 10.
While the AVR extracorporeal blood circuit illustrated in FIGS. 3 and 4, and particularly the use of the AAR method and system, represents a significant improvement in extracorporeal circuits, its implementation can be further refined and improved. A need remains for an AAR system and method that optimizes the air sensor and its functions and that detects and responds to error conditions and faults that can arise over the course of prolonged surgical use.
The typical prior art extracorporeal blood circuit, e.g. the above-described extracorporeal blood circuits of FIGS. 1–3, has to be assembled in the operating room from the above-described components, primed, and monitored during the surgical procedure while the patient is on bypass. This set-up of the components can be time-consuming and cumbersome and can result in missteps that have to be corrected. Therefore, a need remains for an extracorporeal blood circuit having standardized components and that can be set up for use using standardized setup procedures minimizing the risk of error.
The resulting distribution of the components and lines about the operating table can take up considerable space and get in the way during the procedure as described in U.S. Pat. No. 6,071,258, for example. The connections that have to be made can also introduce air leaks introducing air into the extracorporeal blood circuit. A need remains for a compact extracorporeal blood circuit that is optimally positioned in relation to the patient and involves making a minimal number of connections.
The lengths of the interconnected lines are not optimized to minimize prime volume and attendant hemodilution and to minimize the blood contacting surface area. A large blood contacting surface area increases the incidences of embolization of blood cells and plasma traversing the extracorporeal blood circuit and complications associated with immune response, e.g., as platelet depletion, complement activation, and leukocyte activation. Therefore, a need remains for a compact extracorporeal blood circuit having minimal line lengths and minimal blood contacting surface area.
Furthermore, a need remains for such a compact extracorporeal blood circuit with minimal blood-air interfaces causing air to be entrained in the blood. In addition, it is desirable that the components be arranged to take advantage of the kinetic assisted, venous drainage that is provided by the centrifugal venous blood pump in an AVR extracorporeal blood circuit employing an AAR system.
Occasionally, it becomes necessary to “change out” one or more of the components of the extracorporeal blood circuit during the procedure. For example, it may be necessary to replace a blood pump or oxygenator. It may be necessary to prime and flush the newly constituted extracorporeal blood circuit after replacement of the malfunctioning component. The arrangement of lines and connectors may make this very difficult to accomplish. A need therefore remains for a compact extracorporeal blood circuit that can be rapidly and easily substituted for a malfunctioning extracorporeal blood circuit and that can be rapidly primed.
Consequently, a need remains for a extracorporeal blood circuit that is compactly arranged in the operating room, that takes advantage of kinetic assist, and is small in volume to minimize the required prime volume and to minimize the blood contacting surface area and blood-air interfaces. Moreover, a need remains for such an extracorporeal blood circuit that is simple to assemble in relation to other components, that provides for automatic monitoring of blood flow and other operating parameters, that can be simply and rapidly primed, that provides for detection and removal of air from the extracorporeal blood circuit, and that facilitates change out of the extracorporeal blood circuit or components employed with it during the procedure.